xvkbd: fix X.org dependencies

xvkbd is a X.org client application, so there is no reason to depend
on the X.org server. Instead, depend on the X.org client libraries
that xvkbd actually links against.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>